The present invention relates to the technical field of corrosion control, more particularly the corrosion control of metallic substrates during and after temperature exposure.
More particularly the present invention relates to a coating, more particularly a high-temperature corrosion control coating, for generating cathodic high-temperature corrosion protection on a metallic substrate.
The present invention further relates to a first coating composition for producing a cathodic corrosion control layer and to a second coating composition for producing an oxygen barrier layer.
Furthermore, the present invention relates to a method for producing a high-temperature corrosion control coating on a metallic substrate.
Moreover, the present invention relates to a coated metallic substrate which comprises a corrosion control coating.
Moreover, the present invention relates to the use of an oxygen barrier layer for improving the temperature stability of a cathodic corrosion control coating.
Lastly the present invention relates to the use of a coating composition for improving the temperature stability of a cathodic corrosion control coating.
Signs of corrosion on metals are observed across all fields of industry, and are of high significance, since the durability or service life of machines, vehicles, industrial plant, or even buildings is dependent, often decisively so, on the corrosion properties of the metals used. Corrosion means that metal parts must be replaced or renovated, operations which always involve time, materials and costs.
According to DIN EN ISO 8044, corrosion is the physico-chemical interaction between a metal and its environment that leads to a change in the properties of the metal and that can lead to considerable adverse effects on the functions of the metal, its surroundings, or the technical system in which the metals are being used. Metal corrosion generally comprises electrochemical processes, specifically the oxidation of metals by oxygen, optionally in the presence of aqueous electrolyte solutions, with formation of metal oxide layers.
Since corrosion processes often determine the durability or service life of metals or metal components, it is necessary to reduce the corrosion susceptibility and corrosion rate of metals. One way of protecting metals from corrosion is to use passive systems—coatings, for example, such as protective coatings—which are intended to protect the metal from environmental events and hence from corrosion. Another way is to use active systems, where the metal component for protection acts as a cathode—electrochemical processes are being used—and hence oxidation of the metal or metal ions formed are immediately reduced. This cathodic corrosion control may be obtained on the one hand by application of an external electrical voltage; on the other hand, however, it is also possible for the metal component for protection to be brought into contact electrically with a baser metal, i.e. one with a lower electrochemical standard potential. The two metals form an electrochemical system, in which the baser metal represents the anode, referred to as the sacrificial anode, and is oxidized, while the more noble metal is the cathode, at which reduction takes place.
One specific form of cathodic corrosion control is the coating of the metal parts for protection with a metal which is baser by comparison with the metal for protection. One particularly widespread form of the coating of metals, especially of steel sheets, is that of galvanizing. With galvanizing, steel customarily, especially steel sheets, are coated with elemental zinc by being dipped into baths of molten zinc, in a process known as hot-dip galvanizing, to produce sheets of hot dipped galvanized steel—HDGS.
A further, widespread possibility for galvanizing is electrolytic zinc plating or electrogalvanization, in which steel sheets or steel components are coated with a layer of zinc by application of an external voltage in an electrolyte bath comprising zinc ions.
Both aforementioned processes result in uniform, durable zinc coats, which are able to extend significantly the lifetimes of the metal components, but which under certain conditions of application exhibit a series of disadvantages.
In the case of higher-strength steels, the formation of hydrogen in the plating process may be a disadvantage, owing to the possibility of hydrogen embrittlement. Complex geometries or bulk material for coating, in turn, cannot be hot-dip galvanized without disadvantages, since, for example, dished regions would become filled with the zinc, or finely structured surface morphologies would become clogged with zinc.
Furthermore, coatings of pure elemental zinc do not exhibit satisfactory results under temperature load, these unsatisfactory results being blamed on the one hand on the low melting point of zinc, of around 420° C., and on the other hand on the accelerated oxidation of the zinc to zinc oxide by atmospheric oxygen even at temperatures well below the melting point.
In order to avoid at least some disadvantages of hot-dip-galvanized or electrogalvanized metal parts, what are called zinc flake coatings are often employed in practice. Zinc flake coatings contain zinc lamellae, i.e. platelet-shaped zinc pigments, in a predominantly inorganic binder. The mixture of binder and zinc lamellae is applied, customarily in the form of a dispersion, to the metal part for protection, and the binder is subsequently crosslinked to produce an impervious, homogeneous layer having a thickness of 5 to 15 μm. Despite the embedding of the zinc particles into a binder matrix, zinc flake coatings exhibit electrical conductivity and ensure high cathodic protection; in particular, in the salt spray test at the scribe mark in accordance with DIN EN ISO 9227, zinc flake coatings exhibit significantly improved corrosion resistance relative to galvanized or electrogalvanized metal parts.
Zinc flake coatings consist customarily of a predominantly inorganic matrix of silicon dioxide or titanium dioxide, in which the zinc lamellae are embedded. Typical zinc flake coatings, which are applied in the form of the corresponding coating composition, also called zinc flake primer, to a substrate, are described for example in WO 2007/130838 A2.
Under normal conditions, the coatings obtained by electrogalvanizing, hot-dip galvanizing or zinc flake coatings provide a high level of protection against corrosion; at elevated temperatures, however, the cathodic corrosion control afforded by zinc coatings, especially zinc flake coatings, reduces sharply or even breaks down completely even after short temperature exposures. For example, steel panels treated with a zinc flake primer, after undergoing temperature exposure at 200° C. over a period of several hours, or accelerated temperature exposure at 300° C., and subsequently subjected to a salt spray test with scribe mark in accordance with DIN EN ISO 9227, no longer exhibit corrosion control, or at least no longer exhibit sufficient corrosion control, a fact attributable to the oxidation of the zinc to form zinc oxide, which does not provide cathodic protection.
In the prior art, therefore, there has been no lack of attempts to increase the temperature stability of zinc coatings. In some cases, for instance, coatings of zinc alloys containing substantial fractions of manganese are used, in order to permit brief heating of coated steel parts to temperatures of 900° C. for subsequent hot shaping operations, without detriment to the cathodic corrosion control. The use of substantial amounts of manganese, however, makes this process costly. Furthermore, this type of cathodic corrosion control is resistant only to short-term temperature loads. Over the long term, cathodic corrosion control cannot be maintained at elevated temperatures.
The temperature stability of zinc-containing coatings can be obtained by what are called galvannealed processes, in which first a zinc layer is applied to a steel substrate and, by subsequent heat treatment, defined iron-zinc alloys are obtained on the surface of the substrate. Iron-zinc alloys are less sensitive both to temperature load and to mechanical stress than pure zinc coatings, but they rapidly exhibit superficial red rust, necessitating costly and inconvenient cleaning in the case of subsequent coating. In view of the susceptibility to red rust, galvannealed steel substrates have a surface of low aesthetic appeal, thus often ruling out applications in the visible sector without further coating. A further disadvantage of the galvannealed coatings is that the production of the iron-zinc alloy takes place, in a manner which is costly and inconvenient from the standpoints of technology and energy, under an inert gas atmosphere, and the coatings at a high temperature also exhibit a tendency towards increased corrosion, albeit less than in the case of pure zinc.
DE 10 2012 005 806 A1 describes a two-layer coating system, having a first layer, comprising zinc particles, and above said layer a second layer, which comprises aluminium particles. Both layers contain epoxide-modified alkoxysilanes as binders. The intention, through selection of appropriate catalysts, is to achieve simultaneous curing of both layers, thus supposedly producing well-adhering coatings having cathodic corrosion control properties. This coating as well, however, withstands only short-term temperature loads of around 300° C. without the cathodic corrosion control being dramatically reduced or broken down.
In the prior art, therefore, there is a lack of suitable active corrosion control, especially on the basis of a zinc coating, which maintains a cathodic corrosion control effect even in the event of relatively long-lasting temperature loads and subsequent to such temperature loads. A high-temperature-resistant corrosion control coating of this kind would be able for example to achieve significant increases in the durability or service life of metal parts in engines, exhaust systems and screw systems in temperature-exposed areas, energy recovery plants or industrial plants.